The present invention relates in general to pneumatic conveying heaters, and in particular to a method of and a device for increasing the efficiency of heating finely divided solid particles in pneumatic conveying ducts wherein in a known manner a stream of hot conveying gas entrains at a lower zone of the duct dispersed solid particles to convey the same upwardly to a classifying device.
The heat treatment of finely divided solid particles is frequently carried out in the so-called pneumatic conveying heaters. The pneumatic conveying heater consists usually of an upright conveying and heating duct in which the finely divided solid particles are entrained in a stream of a hot gas and conveyed upwardly, whereby a heat transfer from the gas to the solid particles takes place. At the upper end of the conveying duct, the gases and the solid particles are separated from each other, usually by means of a cyclone. Hot gases, also called heat carrying gases or conveying gases, needed for this process are usually generated in a furnace or combustion chamber and are mixed with exhaust gases separated at the upper end of the conveying duct to such a degree that the conveying gas attains the desired temperature and volume. Thereupon, the hot gas is fed into the lower inlet of the upright conveying duct and entrains the solid particles which are dispersed into the duct downstream of the inlet opening.
The conveying ducts of the above described type can be assembled in one stage or in several stages and are applicable for example in drying and preheating of coking coal. In this case, the moisture of the coal is initially reduced at most about 10% and subsequently the coal particles are heated to about 150.degree.-250.degree. C.
The disadvantage of known pneumatic conveying heaters is the fact that the effectiveness of the heat transfer from the conveying gas to the solid particles attains its maximum value only in the acceleration phase of the stream travel, inasmuch as, due to the high speed difference between the heat carrying gas and the solid particles resulting in this acceleration phase, the solid particles are subject to an intensive circumcirculation. After the acceleration phase the effectiveness of the heat transfer sharply drops because of the subsequent very small speed differences. It has been necessary, therefore, to form relatively high pneumatic conveying ducts in order to obtain a sufficient duration of the gas stream action necessary for transferring heat from the carrier gas to the solid particles.